A quantum key distribution system includes a transmitting node, a receiving node, and an optical fiber link that connects the transmitting node and the receiving node. The transmitting node continuously transmits single photons to the receiving node through the optical fiber link (quantum communication channel), which is an optical fiber communication channel. After that, the transmitting node and the receiving node exchange control information to each other, thereby sharing an encryption key between the transmitting node and the receiving node with safety. This technology is achieved by a technology generally called “quantum key distribution (QKD)”.
Quantum key distribution uses the uncertainty principle, which is one of the basic quantum-mechanical principles, that photons used to share an encryption key change their physical state when observed. Based on this principle, if an eavesdropper observes photons that contain encryption key information transmitted from the transmitting node on the quantum communication channel, the physical state of the photons is changed, and the receiving node that has received the photons can detect that the photons have been observed by the eavesdropper. Thus, by exchanging control information between the transmitting node and the receiving node on the basis of a sequence of photons obtained at the transmitting node and a sequence of photons detected at the receiving node, a safe encryption key is finally obtained.
Two nodes (for example, the above-mentioned transmitting node and receiving node) use the above-mentioned shared encryption key to encrypt and decrypt data, and perform encrypted data communication by using an encrypted communication scheme called “one-time pad”. One-time pad is an encrypted communication scheme in which 1-byte data is encrypted with a 1-byte encryption key before transmitted, the data is decrypted with use of the same 1-byte encryption key at the time of reception, and the encryption key used once is discarded. In the encrypted data communication with one-time pad, it is guaranteed based on information theory that data cannot be deciphered by even an eavesdropper who has any knowledge. The technology for encrypted data communication using a safe encryption key shared by quantum key distribution as described above is called “quantum encryption technology”. The quantum key distribution and the quantum encryption technology provide such an extremely high security function.
Achieving the quantum key distribution requires a technology for accurately controlling optical elements used to continuously transmit and receive single photons as described above, and the cost is increased. It is also necessary to lay an optical fiber that connects the transmitting node and the receiving node, and the cost is increased. In terms of cost, it is difficult to install and lay such costly devices and facilities individually for small-scale applications or a plurality of users of a quantum key distribution service or a quantum encryption service. As a technology dealing with the problem, it is proposed to employ an operation system that provides encryption keys for a plurality of applications by using a set of quantum key distribution devices.
The above-mentioned technology, however, simply allocates encryption keys generated and accumulated by a set of quantum key distribution devices to the applications. This technology cannot provide a plurality of different kinds of quantum key distributions at low cost, cannot provide different quantum key distributions or quantum encryption functions using different parameters and variations, such as an algorithm, about the quantum key distribution and the quantum encryption technology for each of the plurality of users, and cannot provide different quantum key distributions or quantum encryption functions according to requests of each of the plurality of users.